This invention relates to a process for catalytic cracking of hydrocarbon feedstocks to produce an enhanced yield of light (C2-C4) olefins and in particular an enhanced yield of propylene.
Catalytic cracking, and particularly fluid catalytic cracking (FCC), is routinely used to convert heavy hydrocarbon feedstocks to lighter products, such as gasoline and distillate range fractions. Conventional processes for catalytic cracking of heavy hydrocarbon feedstocks to gasoline and distillate fractions typically use a large pore molecular sieve, such as zeolite Y, as the primary cracking component. It is also well-known to add a medium pore molecular sieve, such as ZSM-5 and ZSM-35, to the cracking catalyst composition to increase the octane number of the gasoline fraction (see U.S. Pat. No. 4,828,679).
In addition, it is known from, for example, U.S. Pat. No. 4,969,987 to employ medium pore molecular sieves, such as ZSM-5 and ZSM-12, to crack paraffinic and naphthenic naphthas to produce a light olefinic fraction rich in C4-C5 isoalkenes and a C6+ liquid fraction of enhanced octane value.
There is, however, an increasing need to enhance the yield of light olefins, especially propylene, in the product slate from catalytic cracking processes. Thus propylene is in high demand for a variety commercial application, particularly in the manufacture of polypropylene, isopropyl alcohol, propylene oxide, cumene, synthetic glycerol, isoprene, and oxo alcohols.
Co-pending U.S. patent application Ser. No. 09/866,907 describes a synthetic porous crystalline material, ITQ-13, which is a single crystalline phase material having a unique 3-dimensional channel system comprising three sets of channels, two defined by 10-membered rings of tetrahedrally coordinated atoms and the third by 9-membered rings of tetrahedrally coordinated atoms.
According to the present invention, it has now been found that the porous crystalline material, ITQ-13, is effective in producing enhanced yields of propylene, as compared with known intermediate pore molecular sieves, such as ZSM-5, when used to crack naphthas and when used as a additive catalyst in combination with a large pore molecular sieve catalyst in the catalytic cracking of heavier hydrocarbon feedstocks, such as vacuum gas oils.
Thus, in its broadest aspect, the present invention resides in a catalytic cracking process for selectively producing C2 to C4 olefins, the process comprising contacting, under catalytic cracking conditions, a feedstock containing hydrocarbons having at least 5 carbon atoms with a catalyst composition comprising a synthetic porous crystalline material comprising a framework of tetrahedral atoms bridged by oxygen atoms, the tetrahedral atom framework being defined by a unit cell with atomic coordinates in nanometers shown in Table 1 below, wherein each coordinate position may vary within xc2x10.05 nanometer.
Preferably, the synthetic porous crystalline material has an X-ray diffraction pattern including d-spacing and relative intensity values substantially as set forth in Table 2 below.
In one preferred embodiment of the invention, the feedstock comprises a naphtha having a boiling range of about 25xc2x0 C. to about 225xc2x0 C.
In a further preferred embodiment of the invention, the feedstock comprises hydrocarbon mixture having an initial boiling point of at least 200xc2x0 C. and the catalyst composition also comprises a large pore molecular sieve having a pore size greater than 6 Angstrom.